1. Field of the Invention
This invention relates to a rotating electric motor, and more particularly to a rotating electric motor comprising an armature core made of a magnetic material and having a plurality of teeth and a plurality of overlapping winding coils of polyphase winding groups, and a field permanent magnet member which is arranged to have a plurality of alternating N and S poles which are opposed to said teeth of the armature core, the number of N and S poles being P which is an even number, and the number of teeth being T which is an integral multiple of P and is not less than 2P.
2. Description of the Prior Art
A rotating electric motor comprising an armature core of magnetic material having a plurality of teeth and a field permanent magnet member having magnetized poles opposed to the teeth is widely used since it has high efficiency. But in the prior art there is a problem that a harmful vibration occurs due to an intense cogging torque generated by the interaction between the magnetized poles of the permanent magnet and the teeth of the armature core. This cogging torque prevents a smooth rotation of the rotating electric motor. In order to reduce the cogging torque, a skewed armature core is used in some cases. But, it is difficult to make the skewed armature core, and the cogging torque is not adequately reduced. The use of an armature core having no teeth for getting a smooth rotation is not practical because of its low efficiency, which necessitates a large motor size.
A construction of a rotating electric motor with reduced cogging torque is disclosed in the specification of U.S. Pat. No. 4,280,072 or the specification of Japanese Laid-open (unexamined) Patent No. 55-71163. In these patent specifications, indented portions are used to reduce cogging torque. This method is very effective in the case where the number of teeth T is smaller than twice the number P of the N and S poles (that is, T&lt;2P), such as the rotating electric motors shown in FIG. 1, FIG. 5 and FIG. 6 of U.S. Pat. No. 4,280,072, since the face of each of the teeth is wide enough to provide thereon a number of indented portions. But, the cogging torque of a rotating electric motor such as that shown in FIG. 8 of U.S. Pat. No. 4,280,072, in which T is not less than 2P, is not sufficiently reduced by the above method (the same rotating electric motor is disclosed in Japanese Laid-open Patent No. 55-71163).
The conventional rotating electric motor will be described in detail hereinbelow.
FIG. 1 is a schematic sectional view of a conventional rotating electric motor with the relationship T=6P, wherein T is the number of teeth of the armature core and P is the number of pulses of the field permanent magnet. In FIG. 1, a cylindrical permanent magnet 3 is fixed to the outer circumference of a rotor 2 made of a magnetic material, and the permanent magnet 3 rotates with the rotor 2 around a rotary axis (shaft) 1. The permanent magnet 3 has 4 poles of alternating N and S poles positioned apart by equal angles of 90 degrees, that is, P=4. The teeth 6 of an armature core 4, each of which is formed between two adjacent winding slots 5, are faced to the poles of the permanent magnet 3. The rotary shaft 1 of the rotor 2 is rotatably supported by the armature core 4. Therefore, the relative position between the teeth 6 of the armature core 4 and the poles of the permanent magnet 3 changes according to the rotation of the rotor 2.
FIG. 2 shows a development view of the conventional motor of FIG. 1 developed at the lines X--X' and Y--Y', when these lines are in a line. The armature core 4 has 24 winding slots a to x a spaced apart by equal angles of 15 degree, and 24 teeth are provided between two adjacent winding slots, that is, T=24. Overlapping winding coils A1, A2, A3, A4, B1, B2, B3, B4, C1, C2, C3 and C4 are wound in the winding slots a to x. Each of the winding coils A1 to C4 encircles 5 of the teeth of the armature core 4. That is, A1 is wound in the winding slots a and f, A2 is wound in the winding slots g and l, A3 is wound in the winding slots m and r, A4 is wound in the winding slots s and x, B1 is wound in the winding slots e and j, B2 is wound in the winding slots k and p, B3 is wound in the winding slots q and v, B4 is wound in the winding slots w and d, C1 is wound in the winding slots i and n, C2 is wound in the winding slots o and t, C3 is wound in the winding slots u and b, and C4 is wound in the winding slots c and h. The winding coils A1,A2,A3 and A4 are connected in series to form a winding group A of a first phase, the winding coils B1,B2,B3 and B4 are connected in series to form a winding group B of a second phase, and the winding coils C1,C2,C3 and C4 are connected in series to form a winding group C of a third phase. The phase difference among the winding groups A,B and C is equal to 120el degrees (electrical degrees), where 180el is equivalent to 1 pole pitch of 360/P degrees of the permanent magnet 3. In FIG. 1, if P=4, then 180el is equivalent to 90 degrees (mechanical degrees). Therefore, a torque accelerating the rotor 2 is obtained by supplying three phase currents to the three phase winding groups A, B and C.
FIG. 3 is a schematic sectional view of another conventional rotating electric motor with the relationship T=3P. The structure of the conventional motor shown in FIG. 3 is the same as that of the conventional motor shown in FIG. 1, except for the relationship of T and P, and the winding pitch. A cylindrical permanent magnet 13 is fixed to the outer circumference of a rotor 12 made of a magnetic material, and the permanent magnet 13 rotates with the rotor 12 around a rotary shaft 11. The permanent magnet 13 has 4 poles of alternating N and S poles positioned apart by equal angles of 90 degrees, that is, P=4. The teeth 16 of an armature core 14, each of which is formed between two adjacent winding slots 15, are faced to the poles of the permanent magnet 13. The rotary shaft 11 of the rotor 12 is rotatably supported by the armature core 14. Therefore, the relative position between the teeth 16 of the armature core 14 and the poles of the permanent magnet 13 changes according to the rotation of the rotor 12.
FIG. 4 shows a development view of the conventional motor of FIG. 3 developed at the lines X--X' and Y--Y', when these lines are in a line. The armature core 14 has 12 winding slots a to l spaced apart by equal angles of 30 degrees, and 12 teeth are provided between two adjacent winding slots, that is, T=12. Overlapping winding coils A1, A2, A3, A4, B1, B2, B3, B4, C1, C2, C3 and C4 are wound in the winding slots a to l. Each of the winding coils A1 to C4 encircles 3 of the teeth of the armature core 14. That is, A1 is wound in the winding slots a and d, A2 is wound in the winding slots d and g, A3 is wound in the winding slots g and j, A4 is wound in the winding slots j and a, B1 is wound in the winding slots c and f, B2 is wound in the winding slots f and i, B3 is wound in the winding slots i and l, B4 is wound in the winding slots l and c, C1 is wound in the winding slots e and h, C2 is wound in the winding slots h and k, C3 is wound in the winding slots k and b, and C4 is wound in the winding slots b and e. The winding coils A1,A2,A3 and A4 are connected in series to form a winding group A of a first phase, the winding coils B1,B2,B3 and B4 are connected in series to form a winding group B of a second phase, and the winding coils C1,C2,C3 and C4 are connected in series to form a winding group C of a third phase. The phase difference among the winding groups A,B and C is equal to 120el degrees. In FIG. 3, if P=4, then 180el degrees is equivalent to 90 degrees (mechnical degrees). Therefore, a torque accelerating the rotor 2 is obtained by supplying three phase currents to the three phase winding groups A, B and C.